Enantiornithes are the most successful clade of Mesozoic birds. Here, we describe a new enantiornithine bird, Cruralispennia multidonta gen. et sp. nov., from the Protopteryx-horizon of the Early Cretaceous Huajiying Formation of China. Despite being among the oldest known enantiornithines, Cruralispennia displays derived morphologies that are unexpected at such an early stage in the evolution of this clade. A plough-shaped pygostyle, like that of the Ornithuromorpha, evolved convergently in the Cruralispennia lineage, highlighting the homoplastic nature of early avian evolution. The extremely slender coracoid morphology was previously unknown among Early Cretaceous enantiornithines but is common in Late Cretaceous taxa, indicating that by 131 million years ago this clade had already experienced considerable morphological differentiation. Cruralispennia preserves unusual crural feathers that are proximally wire-like with filamentous distal tips, a new morphotype previously unknown among fossil or modern feathers, further increasing the known diversity of primitive feather morphologies.

Systematic palaeontology

Aves Linnaeus 1758

Ornithothoraces Chiappe 1995

Enantiornithes Walker 1981

Cruralispennia multidonta gen. et sp. nov.

Etymology. The generic name is derived from Latin ‘Cruralis’ and ‘penna’, referring to the unique feathers on the tibiotarsus; the specific name is derived from Latin ‘mult’ and ‘donta’, referring to the numerous dentary teeth.

Holotype. IVPP 21711 (housed at the Institute of Vertebrate Paleontology and Paleoanthropology), a nearly fully articulated partial skeleton with associated feathers preserved on a single slab.

Locality and horizon. The new specimen is collected from the Protopteryx-horizon of the Huajiying Formation at the Sichakou Basin, Fengning County, Hebei Province, northeastern China. Four other birds are reported from the same horizon, Eoconfuciusornis, Protopteryx, Eopengornisand Archaeornithura. Stratigraphic correlation and isotopic dating place this horizon at 130.7 Myr ago, late Early Cretaceous.

Diagnosis.A small enantiornithine with the following unique features: 14 dentary teeth; abbreviated, plough-shaped pygostyle with a pygostyle/tarsometatarsus length ratio of about 0.28; coracoid mediolaterally narrow with the sternal margin measuring only one-quarter of the proximo-distal length; sternum bearing a V-shaped caudal margin and two pairs of subequal caudal trabeculae; manus shorter than the humerus; postacetabular process of the ilium short and strongly ventrally directed; dorsal process of the ischium more distally placed; and pubis without a distal expansion.

Figure 7: Major transitions of fibula and tail morphology across Mesozoic birds.
The tree, simplified from the strict consensus tree produced by our phylogenetic analysis, is time-scaled using the ‘minimal branch length’ method with a minimum branch length of 1 Myr (see Supplementary Fig. 5 for complete result and node support values). The thick lines indicate the temporal range of fossil taxa. An elongated fibula (in red) is developed in non-ornithothoracine Aves, and the most basal enantiornithines Protopteryx and Pengornithidae; a reduced fibula is convergently evolved in the Ornithuromorpha and derived enantiornithines. Cruralispennia preserves a plough-shaped pygostyle (in pink), which has long been considered unique to Ornithuromorpha (the pink branches). The apparent absence of tail fanning in Cruralispennia indicates that the plough-shaped pygostyle and tail fanning is evolutionarily decoupled in this lineage (the silhouettes were from ref. 47; the reconstruction of tail feathers is on basis of refs 3, 30).